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CHAPTER 2: LITERA TURE STUDY 42 compressor where it is compressed to a high temperature and high pressure gas before entering the gas cooler (Fartaj et a/., 2004). From the results of Sarkar (2007) it was noted that there is an increase in system COP of up to 75% when the evaporation temperature is varied from -10°C to 10°C. It was also found that the system COP increases sharply as the gas cooler outlet temperature decreases. For an evaporating temperature of O°C the system COP almost doubles as the gas cooler exit temperature decreases from 50 to 35°C, (Sarkar, 2007). Therefore it could be seen that the system COP is dependent on the evaporation temperature, compressor efficiency, gas cooler outlet temperature, compressor discharge temperature and the heat exchanger effectiveness, and as a result attention should be given to these factors. From the results mentioned above the two factors that a designer has to take in account is to design a CO2 system with the lowest possible gas cooler outlet temperature and the highest possible evaporation temperature. This is highly relevant for South African conditions since the ambient temperatures are usually high resulting in heat pump systems having a high evaporation temperature. It is only the gas cooler outlet temperature that would have to be looked at closely since the water inlet temperatures are also usually higher because of the high ambient temperatures. According to "findings made by Sarkar et al. (2006), CO2 systems can effectively be used in plants where temperatures of between 100°C and 140°C are required. Typically other refrigerants are not capable of reaching these temperatures. Although reaching these temperatures would lead to less efficient cycles, the high temperatures reached are worthwhile for some applications. Constraints that arise from the very high operating pressure make it hard to design and manufacture basic components, but the industry is starting to solve these problems. The high density of CO2 helps manufacturers to find solutions since the high density allows the use of smaller components (Cecchinato et al., 2005). The storage tank for the hot water is also an important factor with regard to system efficiency. Good stratification is difficult to be obtained and this increases the inlet water temperature to the system thus decreasing system performance and also increasing the water flow rate, as illustrated in Figure 2.27. Therefore stratification becomes mandatory when high energy efficiency is required for heat pumps using CO2 as refrigerant (Cecchinato et al., 2005). Because of the high operating pressures the pressure drop in the heat exchangers of a CO2 system is lower than for a conventional refrigerating system (Cecchinato et al., 2005). A Techno-Economical Analysis of a COl Heat Pump. School ofMechanical Engineering, North-West UniversityPDF Image | CO2 HEAT PUMP Analysis
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